1. Field of the Invention
The present invention relates to a multilayer ceramic substrate, a method for producing a multilayer ceramic substrate, and an electronic component including a multilayer ceramic substrate. In particular, the present invention relates to a multilayer ceramic substrate having a laminated structure including surface portions and an inner layer portion, in which each of the surface portions has a thermal expansion coefficient less than that of the inner portion, so that the multilayer ceramic substrate has improved strength.
2. Description of the Related Art
A multilayer ceramic substrate is described in, for example, Japanese Unexamined Patent Application Publication No. 6-29664. Japanese Unexamined Patent Application Publication No. 6-29664 discloses a low-temperature co-fired multilayer ceramic substrate including glass and a crystalline material, in which surface portions have a thermal expansion coefficient less than that of an inner layer portion, and in which the total thickness of the surface portions arranged on both sides thereof is less than the thickness of the inner layer portion. Japanese Unexamined Patent Application Publication No. 6-29664 discloses that the use of such a structure generates compressive stresses in both surface portions during a cooling step after firing, thereby improving the transverse strength of the multilayer ceramic substrate.
The inventor of the present invention discovered that when a glass-ceramic material including an MO—SiO2—Al2O3—B2O3-based glass (wherein MO is at least one selected from CaO, MgO, SrO, and BaO) and an alumina powder is used as a material defining surface portions of a multilayer ceramic substrate as described above, the multilayer ceramic substrate has an improved transverse strength while having outstanding electrical properties.
However, the multilayer ceramic substrate includes conductive patterns having via conductors arranged so as to pass through the surface portions. It was discovered that when the via conductors arranged so as to pass through the surface portions are made of a low-resistivity Ag-based material and when the surface portions are made of a glass-ceramic material, Ag diffuses into the surface portions to form voids around the via conductors. It was also discovered that, in particular, when the glass-ceramic material including an MO—SiO2—Al2O3—B2O3-based glass and the alumina powder is used, Ag readily diffuses into the surface portions.